1. Field of the Invention
This invention relates to an ink jet printing head for a color printer with which it is possible to obtain a large range of print densities.
2. Description of the Related Art
Examples of conventional ink jet printing heads are provided in U.S. Pat. Nos. 4,490,728, 4,317,124, and 4,611,219. In these patents the printing head includes a stationary nozzle and a stationary thermal element having a plurality of thermal dots each of which has the same dimensions.
Ink is expelled from the nozzle in a jet by bubble pressure created by heating of the ink by the thermal dot element while the nozzle and thermal dot element are in stationary contact.
There are problems associated with the conventional type of construction described in the above patents:
With a one-to-one correspondence between a nozzle and a thermal dot, an ink stoppage in any one nozzle results in no ink jet from that nozzle, and a misprint occurs in the form of a white spot on the paper in front of the nozzle.
Furthermore, if the bubble remains in the nozzle and no ink reaches the thermal dot element due to an ink stoppage in the nozzle, there is a possibility of damage of the thermal dot element.
Also, for a 1:1 correspondence between nozzles and thermal dot elements, it is very difficult to make a nozzle assembly having many nozzles; therefore, it is difficult to obtain a high-density printing condition for full color printing with this type of construction. Conventionally, only simple color printing is obtained with a 1:1 ratio between nozzles and thermal dot elements.